Organic light emitting diodes, which are a self-luminous device, are advantageous because of a wide viewing angle, high contrast, fast response time, high luminance, superior driving voltage, excellent response rate, and polychromatic properties.
Typical organic light emitting diodes are configured to include an organic emission material layer for emitting light, and a first electrode and a second electrode disposed on both sides of the organic emission material layer so as to face each other.
Such organic light emitting diodes are classified into a bottom emission type and a top emission type depending on the direction of light emitted from the organic emission material layer. A bottom emission type organic light emitting diode for emitting light to the substrate is configured such that a reflective electrode is formed on an organic emission material layer, and a transparent electrode is formed under the organic emission material layer. As such, when the organic light emitting diode operates in an active matrix mode, light does not pass through the portion thereof on which a thin film transistor is formed, thus reducing the area where light is transmitted. On the other hand, a top emission type organic light emitting diode is configured such that a transparent electrode is formed on an organic emission material layer and a reflective electrode is formed under the organic emission material layer, thus emitting light to a direction opposite the substrate, and thereby the area where light is transmitted is enlarged, resulting in high luminance.
The bottom emission type organic light emitting diode is configured such that an anode is formed on a substrate, and a hole transport layer, an emission material layer, an electron transport layer, and a cathode are sequentially formed on the anode. As such, the hole transport layer, the emission material layer, and the electron transport layer are organic thin films made of an organic compound.
The cathode comprises a metal layer having properties of a reflective layer, so that light generated from the emission material layer is reflected to the anode layer, thereby increasing luminous efficiency.
The driving principle of the organic light emitting diode thus configured is as follows. When voltage is applied between the anode and the cathode, holes injected from the anode are moved to the emission material layer via the hole transport layer, and electrons injected from the cathode are moved to the emission material layer via the electron transport layer. The carriers such as holes and electrons are re-combined in the emission material layer to form excitons. While these excitons return to a ground state from an excited state, light is produced.
As such, the generated light travels linearly to an anode direction, a cathode direction, and the other directions. The light traveling linearly to the anode is escaped to the air layer through glass, and the light traveling linearly to the cathode is reflected from the metal layer that is the cathode, and then goes again to the anode.
In this regard, Korean Patent Application Publication No. 10-2006-0095489 discloses an organic light emitting diode, which is configured such that an emission material layer is interposed between a first electrode and a second electrode, and a reflective layer for reflecting light emitted from the emission material layer to travel toward the second electrode is formed on the first electrode. Also, Korean Patent Application Publication No. 2001/0101640 discloses a technique for increasing luminous efficiency by determining the film thickness between a light-transmitting electrode and a reflective electrode so as to resonate the desired wavelength using interference caused by multiply reflecting light between the light-transmitting electrode and the reflective electrode.
FIG. 1 illustrates an organic light emitting diode manufactured by a conventional technique. Such a conventional technique is specified below.
In order to manufacture an organic light emitting diode, a soda-lime or alkali-free glass substrate 10 is coated with a transparent conductive film 20 (ITO), after which a photoresist (PR) is applied thereon using a spin coater, followed by UV exposure, thereby forming a desired pattern. Thereafter, the device is loaded on a vacuum deposition machine, and a hole injection layer (HIL) 30, a hole transport layer (HTL) 40, an emission material layer (EML) 50, an electron transport layer (ETL) 60 and a cathode (a metal electrode) 70 are deposited.
Then when direct-current power or voltage ranging from ones to tens of V is applied to the transparent electrode and the metal electrode to allow current to flow, the organic light emitting diode emits light, and light irradiated toward the cathode is reflected through a reflective plate, and is then irradiated toward the glass substrate.
As such, the reflected light may exhibit an interference effect with light that travels toward the anode from the emission material layer, but conventional organic EL (electroluminescent) devices have low constructive interference effects due to structural limitation thereof, making it impossible to obtain high color coordinates. To obtain color coordinates corresponding to high color quality of the organic light emitting diode, proper color coordinates may be ensured by using a material having low color coordinates or by adjusting the device thickness, but driving voltage, efficiency and lifetime may deteriorate undesirably.